1. Field of the Invention
This invention relates to a plunging constant velocity ratio universal joint, comprising:
an outer joint member having an internal cylindrical surface provided with a number of tracks spaced circumferentially about a rotational axis of the joint member;
an inner joint member provided with a number of tracks spaced circumferentially about a rotational axis of the inner joint member, facing the tracks in the outer joint member in pairs;
a plurality of balls for torque transmission between the joint members, disposed one in each facing pair of said tracks;
and a cage having an axis and disposed between the joint members and holding the balls so that their centres lie in a ball centre plane, the cage having a first control surface which is in sliding engagement with the internal cylindrical surface of the outer joint member, and is a surface of revolution about the cage axis of a curved line with a centre of curvature axially offset from the ball centre plane, the cage further having a second control surface which is in sliding engagement with a surface associated with the inner joint member and is a surface of revolution about the cage axis of a curved line with a centre of curvature offset from the ball centre plane on the opposite side thereof from the first control surface, the said offset of the first control surface being greater than the said offset of the second control surface.
Such a plunging constant velocity ratio universal joint will hereafter be referred to as a joint of the kind specified. The effect of the offset of the first and second control surfaces of the cage from the ball centre plane is that, when the joint is articulated, the ball centre plane is caused to bisect the angle between the rotational axes of the joint members thereby making the joint a constant velocity ratio joint.
2. Description of Prior Art
A joint of the kind specified is disclosed in DE-PS-2323822 (GB 1431135). The unequal offset of the centres of curvature of the first and second control surfaces of the cage from the ball centre plane is provided to compensate for an effect which would occur as follows if they were equally offset. When the joint is articulated and is transmitting torque, the balls received in tracks which diverge from one another experience an axial thrust which tends to displace them in the direction in which the tracks diverge, so that they lose the ability to take full part in the transmission of torque. When the centres of curvature of the control surfaces of the cage are unequally offset, the cage is itself subjected to an increased turning moment which counteracts the axial thrust exerted on the balls as above described, so that all the balls take full part in torque transmission. A disadvantage of such a joint is that the difference between the offsets of the control surfaces of the cage has to be chosen as a compromise. If it is too small, the effect at small angles of articulation, i.e. the main operating range of the joint, is insufficient to counteract the above described loss in torque transmitting capacity, whereas, if the difference between the offset distances is too great, the effect is to cause the balls to become jammed at large angles of joint articulation, thereby limiting the operating range of the joint.
Further, with a known joint of the kind specified as above referred to, the cage is freely movable axially of the joint with the result that, in use, the cage tends to migrate to one or other end of its range of axial travel relative to the joint members. When this happens, the joint tends to transmit axial vibrations, which in theory the joint should not do because of its plunging ability.